Gas flow makes electricity

Researchers from the Indian Institute of
Science have found that electricity is produced when gas flows over semiconductor
materials and carbon nanotubes at speeds as low as a few meters per second.

Semiconductors are materials that allow electric current to flow
in variable amounts depending and conditions like the presence of an electrical
field. Carbon nanotubes are rolled-up sheets of carbon atoms that can
be narrower than one nanometer in diameter. Some types of carbon nanotubes
are semiconducting.

The phenomenon could be used to make small, inexpensive gas flow
sensors with no moving parts. The sensors would measure the velocity and
direction of gas flow, which promises to be useful in turbulence and aerodynamics
monitoring and research.

The researchers' device works for several reasons: gas flow on
inclined solids results in a pressure difference along streamlines, an
effect known as the Bernoulli principle; this pressure difference results
in a temperature gradient along a material; the temperature difference
leads to a voltage difference across the material, a principle known as
the Seebeck effect.

The researchers produced a voltage difference of 650 microvolts
and a power flow of 43 nanowatts when they released compressed air over
a small piece of germanium. It's not clear whether the effect could be
used to generate substantial amounts of electricity, but if it could be
scaled up it could serve as an electricity generator with no moving parts,
similar to thermoelectric and piezoelectric generators.

The method could be used to make accurate gas flow sensors in
less than two years, according to the researchers. The work appeared in
the August 20, 2004 issue of Physical Review Letters.